In recent years, with an increase of environmental engineering, development of power generation technologies (e.g., solar photovoltaic power generation) which pose fewer burdens on the environment than conventional power generation methods has been actively conducted. Concurrently with the development of power generation technology, development of power storage technology has also been underway.
As a power storage technology, for instance, there is a power storage device such as a lithium ion secondary battery. Lithium ion secondary batteries are widely prevalent since their energy density is high and because they are well suited for miniaturization.
As a positive electrode active material of a lithium on secondary battery, for example, there are a composite oxide containing at least alkali metal and transition metal and a composite oxide containing at least alkaline earth metal and transition metal. Specifically, an iron phosphate compound (AFePO4, where A is alkali metal such as lithium (Li) or sodium (Na) or alkaline earth metal such as magnesium (Mg) or calcium (Ca)) having an olivine structure can be given.
For example, in lithium iron phosphate (LiFePO4) having an olivine structure, lithium ions are arranged one-dimensionally in <010>, <001>, and <101> directions in such a manner that the ions are connected to each other. In lithium iron phosphate having an olivine structure, lithium ions move in these directions to leave and enter the lithium iron phosphate, whereby charging and discharging are performed. Lithium ions move in a <010> direction more easily than in the other two directions. It is known that lithium iron phosphate (LiFePO4) having an olivine structure possesses favorable characteristics owing to its lithium arranged one-dimensionally and has excellent safety because it does not release oxygen even when heated to higher than 350° C.
A positive electrode active material layer which contains an iron phosphate compound (AFePO4) having an olivine structure as an active material is formed in such a manner that particles of lithium iron phosphate are mixed with a conductive agent (e.g., carbon) and a binder and the mixture is shaped over a current collector (for example, see Patent Document 1). Generally, when a ratio of a material which ions can leave and enter (i.e., an active material; here, lithium iron phosphate) is increased in an active material layer with the volume of the active material layer unchanged, the amount of ions capable of leaving and entering the active material is increased, which can lead to an increase in capacity of a battery.